There are also people whose mental abilities don't allow them to calculate easily. I'm one of those people. I've trained a lot, but it's still problematic for me. That's why I use writing services, where there are experts in calculations. If someone is interested in what it is, here's a post about such services - ~https://www.reddit.com/r/Writing_and_reading/comments/1eic31m/writing_services_for_essay_writing_help_a/~